Optical filtering device including Fabry-Perot cavities and structured layer belonging conjointly to first and second interference filters

ABSTRACT

A filtering device comprising first and second interference filters each comprising a Fabry-Perot cavity formed by semi-reflective layers between which a structured layer is arranged, wherein the structured layer belongs conjointly to the two filters, has a substantially constant thickness, is substantially planar and comprises two materials with different refractive indices arranged in each of the cavities, forming vertical structurings, the cavity of the second filter comprises a spacer arranged between one of the semi-reflective layers and the structured layer so that a distance between the semi-reflective layers of the cavity of the second filter is greater than a distance between the semi-reflective layers of the cavity of the first filter, and the filters comprise a second structured layer arranged in the cavities of the filters, and/or each filter comprises a second Fabry-Perot cavity comprising a third structured layer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.15/308,448, filed Nov. 2, 2016, which is a National Stage ofInternational Application No. PCT/EP2015/059743, filed May 4, 2015,which claims priority to France Application No. 1454082, filed May 6,2014. The entire contents of each of which are incorporated herein byreference.

TECHNICAL FIELD AND PRIOR ART

The invention relates to an optical filtering device comprisinginterference filters with Fabry-Perot cavities, advantageously used inthe field of multispectral or hyperspectral imaging.

An image sensor conventionally comprises a matrix of filters centred onwavelengths different from one another. In a conventional colour imagesensor, this matrix of filters is a Bayer matrix formed by red, greenand blue filters that make it possible to reconstitute the colorimetryof the scene. These filters consist of coloured resins pixelateddirectly on the sensor.

In a hyperspectral camera, filters are also organised in a matrix butare present in larger numbers (typically 5 to 10 or even more), so as todetect the spectral signature of objects in a scene. The informationrestored by this type of camera is richer and the applications arenumerous in industrial vision, or in the military or environmentalfield, for example for detecting gases. The filters are not producedwith coloured resins, in particular because it is difficult to procureresins centred on wavelengths other than those of the red, green andblue colours, and also because the spectral responses of the resins aretoo broad compared with those sought for the filters of a hyperspectralcamera.

A wheel with interference filters, arranged in front of the sensor, andtherefore not integrated therein, is for example used in a hyperspectralcamera. At each acquisition, a different filter is arranged in front ofthe sensor. The various acquisitions made are next combined in order toobtain the final image.

Apart from the drawback related to the non-integration of the filterswith the sensor, given that various acquisitions are necessary forobtaining the filtered images according to the various spectralresponses of the filters located on the wheel, this technology haslimitations for real-time applications.

Wavelength filtering can be done by means of interference filters of theFabry-Perot type, or Fabry-Perot cavity filters. The principle of suchfilters is for example described in the work by H. A. MacLeod, “Thinfilm optical filters III”, Institute of Physics Publishing, London,2001, pages 260-263. A Fabry-Perot cavity comprises two semi-reflectivelayers, or semi-reflective mirrors, arranged facing each other andbetween which there is a medium with a refractive index, or opticalindex, n, for example a layer with a refractive index n. The incidentlight is reflected by the filter for all the wavelengths, except for adiscrete set of wavelengths that are transmitted outside the filter. Forthese transmitted wavelengths, the optical path travelled by the lightin an outward and return travel in the Fabry-Perot cavity is a multipleof 2π. In normal incidence, these wavelengths, or centre wavelengths ofthe spectral responses of the filters, are therefore a function of therefractive index n and of a thickness d of the layer of materialarranged between the two semi-reflective layers. The centre wavelengthsλ_(m), at the various orders m of the Fabry-Perot cavity are defined bythe following equation:

$\begin{matrix}{\lambda_{m} = \frac{2{nd}}{m + \frac{\left( {\Phi_{a} + \Phi_{b}} \right)}{2\pi}}} & (1)\end{matrix}$with m being the order of the Fabry-Perot cavity in question, and Φ_(a)and Φ_(b) being the phase differences occurring in the cavity duringreflections on the semi-reflective layers.

The documents WO 2013/064511 A1 and “Monolithic integration of flexiblespectral filters with CMOS image sensors at wafer level for low costhyperspectral imaging” by M. Jayapala et al., IISW2013, describe filterswith Fabry-Perot cavities wherein the tunability for wavelength isachieved by varying the thickness of the cavity of each of the filters.This is a so-called “staircase” filter technology. Sixteen to thirty-twofilters are thus produced in a sensor suitable for the range 600 nm-1000nm, thus covering a small part of the visible range and the nearinfrared range. The layers of material arranged between thesemi-reflective layers comprise amorphous silicon or SiO₂. Thesemi-reflective layers consist of stacks of alternate layers ofamorphous silicon and SiO₂ common to all the filters. The lower part ofthe visible range, that is to say the wavelengths between approximately400 nm and 600 nm, is inaccessible to these filters because of thecoefficient of absorption of amorphous silicon, which is too great atthese wavelengths. The integration of the filters is monolithic, that ismade directly on the sensor by deposition and etching steps followingthe back-end steps of the sensor.

This type of filtering device has two drawbacks:

-   -   the number of etching steps to be carried out increases with the        number of filters required, whatever the production method        envisaged for this staircase structure;    -   the most economical production method in terms of number of        etching steps (N etching steps for 2^(N) filters) consists of        carrying out successive partial etchings in the same layer.        However, in a batch manufacturing method (to the wafer scale),        the deposition and partial etching steps are always affected by        a certain degree of non-uniformity over the surface of the        wafer, the diameter of which is 200 mm or 300 mm. The centre        wavelength of the spectral responses of the filters is very        sensitive to the thickness of the cavity. The document WO        2013/064511 A1 states that control of the thickness of ±2% is        essential. This type of filtering device is therefore not ideal        for industrial production requiring batch manufacture of the        filters, in particular since the errors caused by the successive        etching steps may be cumulative.

The document U.S. Pat. No. 7,759,679B2 describes a filtering device inwhich the tunability of the filters for wavelength is achieved byvarying the effective refractive index of the medium located between thetwo semi-reflective layers of the cavity, the thickness of this mediumbeing constant in all the filters. For this purpose, nanostructures areetched in a layer comprising a first dielectric material. The etchedzones are filled with a second dielectric material, the refractive indexof which is for example less than that of the first dielectric material,and a chemical mechanical polishing (CMP) is carried out in order tosmooth the layer comprising the nanostructures. The light passingthrough these nanostructures sees a mean refractive index, or effectiverefractive index, the value of which is between those of the refractiveindices of the two dielectric materials since the lateral dimensions ofthe nanostructures are smaller than the wavelengths intended to betransmitted by the filter. Using a single mask, it is possible to varythe dimensions of these nanostructures in the plane of the layer andtherefore to vary the effective refractive index along this structuredlayer in a range lying between the refractive index of the seconddielectric material and the refractive index of the first dielectricmaterial. All the filters required in the structured layer can thereforebe achieved with the implementation of a single lithography and etchingstep. The method for producing this filtering device does not comprisepartial etching and therefore does not have the drawbacks related to theproduction of “staircase” filters.

However, the spectral range accessible for such a filtering device islimited by the difference between the refractive indices of thematerials used for producing the structured layer. However, in thevisible and near infrared ranges, there do not exist very greatdifferences in index between the various known materials that can beused. Thus, for the first order of the Fabry-Perot cavities, it ispossible to tune these filters between 450 nm and 680 nm only, usingSiO₂ (refractive index equal to 1.47) as the low-index material andmaking it possible to produce a filter the spectral response of which iscentred at 450 nm when this filter comprises only SiO₂, and TiO₂(refractive index equal to 2.25) as a high-index material and making itpossible to produce a filter the spectral response of which is centredat 680 nm when this filter comprises only TiO₂. Such a filter devicetherefore does not make it possible to achieve multispectral filteringover all the visible and near infrared ranges simultaneously.

The document US 2011/0290982 A1 describes a filtering device in whichthe same structure principle is used for tuning the filters by theeffective refractive index of the layer of materials located between thesemi-reflective layers. Better selectivity, and in particular betterrejection of the filters, is obtained by means of the use of twoFabry-Perot cavities placed one on top of the other for each of thefilters. To facilitate the production of the patterns in the case wherethe nanostructures are small compared with the accessible resolution inlithography, it is proposed in this document to produce, in place ofvery fine patterns with etching throughout the thickness of thestructured layer, patterns that are wider but over a smaller depth so asto obtain the same effective index. The volume proportions of the twomaterials are in these cases the same as for etching throughout theentire thickness of the material for very fine patterns. The physicalthickness of the cavity, defined by the distance between thesemi-reflective layers corresponding to metal layers, also remainsconstant.

As with the document U.S. Pat. No. 7,759,679B2, the spectral rangeaccessible with this type of device is limited by the difference inindex between the two dielectric materials used. In addition, whennanostructures are produced through only part of the thickness of thelayer, it is necessary, to achieve the same wavelength ranges, toproduce two layers nanostructured one on the other, inside each of theFabry-Perot cavities. This increases the complexity and cost ofproducing the filtering device since each nanostructured layer requiresa high-resolution lithography step. Moreover, etching the secondnanostructured layer may, by over-etching, degrade the firstnanostructured layer with such a method.

DISCLOSURE OF THE INVENTION

One aim of the present invention is to propose an optical filteringdevice solving at least some of the problems of the filtering devices ofthe prior art disclosed above.

For this purpose, the present invention proposes an optical filteringdevice comprising at least first and second interference filters eachcomprising at least one first Fabry-Perot cavity formed by first andsecond semi-reflective layers between which at least one firststructured layer is arranged, in which:

-   -   the first structured layer belongs conjointly to the first and        second interference filters, has a substantially constant        thickness, is substantially planar and comprises first portions        of at least two dielectric or semiconductor materials, with        different refractive indices, arranged, in each of the first        Fabry-Perot cavities and in a plane parallel to the first        semi-reflective layer, alongside one another in alternation;    -   the first Fabry-Perot cavity of the second interference filter        comprises at least one first spacer arranged between one of the        first and second semi-reflective layers and the first structured        layer in such a way that a distance between the first and second        semi-reflective layers of the first Fabry-Perot cavity of the        second interference filter is greater than a distance between        the first and second semi-reflective layers of the first        Fabry-Perot cavity of the first interference filter;

and in which the first and second interference filters are producedaccording to a first configuration and/or a second configuration suchthat:

-   -   according to the first configuration, the device further        comprises a second structured layer arranged between the first        and second semi-reflective layers, belonging conjointly to the        first and second interference filters, having a substantially        constant thickness, being substantially planar and comprising        second portions of the two materials with different refractive        indices arranged, in each of the first Fabry-Perot cavities and        in the plane parallel to the first semi-reflective layer,        alongside one another in alternation;    -   according to the second configuration, the first and second        interference filters each comprise at least one second        Fabry-Perot cavity arranged on top of the first Fabry-Perot        cavity and formed by the first and a third semi-reflective layer        between which at least one third structured layer is arranged,        the third structured layer belonging conjointly to the first and        second interference filters, having a substantially constant        thickness, being substantially planar and comprising third        portions of the two materials with different refractive indices        arranged, in each of the second Fabry-Perot cavities and in the        plane parallel to the first semi-reflective layer, alongside one        another in alternation, the second Fabry-Perot cavity of the        second interference filter further comprising at least one        second spacer arranged between the third semi-reflective layer        and the third structured layer so that a distance between the        first and third semi-reflective layers of the second Fabry-Perot        cavity of the second interference filter is greater than a        distance between the first and third semi-reflective layers of        the second Fabry-Perot cavity of the first interference filter.

An optical filtering device is also described, comprising at least firstand second interference filters each comprising at least one firstFabry-Perot cavity formed by first and second semi-reflective layersbetween which at least one first structured layer is arranged, in which:

-   -   the first structured layer is common to the first and second        interference filters,    -   the first structured layer has a substantially constant        thickness,    -   the first structured layer comprises at least two materials with        different refractive indices included in each of the first        Fabry-Perot cavities, and    -   the first Fabry-Perot cavity of the second interference filter        comprises at least one first spacer arranged between one of the        first and second semi-reflective layers and the first structured        layer so that a distance between the first and second        semi-reflective layers of the first Fabry-Perot cavity of the        second interference filter is greater than a distance between        the first and second semi-reflective layers of the first        Fabry-Perot cavity of the first interference filter.

The expression “structured layer” designates the fact that the layercomprises portions of the two materials with different refractiveindices that form structurings, or patterns, in the layer. In such astructured layer, portions of the first of the two materials andportions of the second of the two materials are arranged, in a planeparallel to the main faces of this layer (a plane that is alsoperpendicular to the direction of stacking of the layers of the device),alongside one another in alternation, that is to say so that a portionof the first of the two materials is arranged between at least twoportions of the second of the two materials and so that a portion of thesecond of the two materials is arranged between at least two portions ofthe first of the two materials. Such structurings do not constituteroughnesses or a superimposition of layers of these two materials. Suchstructurings can be seen as transverse, or vertical, structures arrangedalongside one another in the plane parallel to the main faces of thestructured layer.

Such an optical filtering device therefore proposes the production of aplurality of interference filters with Fabry-Perot cavities the centrewavelengths of which are defined by a plurality of parameters related tothe structured layer (the values of the refractive indices of the twomaterials used, the parameters, such as the form and dimensions, of thepatterns formed by the two materials, the thickness of the structuredlayer) but also, for the second interference filter or filters, by aplurality of parameters related to the spacer present in the Fabry-Perotcavity or cavities of the second interference filter or filters (therefractive index of the material of the spacer, its thickness, etc.).

Thus, compared with the filtering devices of the prior art having onlyone structured layer with a constant thickness, the optical filteringdevice according to the invention is suitable for carrying out filteringin a more extensive range of wavelengths, which may cover for examplethe visible range and the near infrared, and in particular which is notlimited by the nature of the materials used for producing the structuredlayer, by virtue of the presence of the spacer in the secondinterference filter or filters.

Compared with a staircase optical filtering device, the opticalfiltering device according to the invention may be produced with a muchlower number of etching steps, without having recourse to partialetching steps. The optical filtering device according to the inventionis therefore well suited to collective manufacture of staircase filtersto the substrate, or wafer, scale without errors caused by successivepartial etching steps.

The optical filtering device according to the invention also comprises astructure suitable for integration on a sensor, for example an imagesensor of the CMOS type, allowing real-time capture throughout the rangeof spectral responses of the interference filters of the opticalfiltering device.

Each of the interference filters can transmit a single spectral band(each interference filter therefore being of the band-pass type), forexample in the complete visible and near-infrared range, thusfacilitating processing of the image captured via a sensor provided withsuch an optical filtering device.

The optical filtering device may cover solely the visible range, forexample when the materials that can be used for forming the structuredlayer have refractive indices with similar values. In anotherconfiguration, the optical filtering device may cover at least part ofthe visible range and/or at least part of the infrared range (nearinfrared and/or mid-infrared and/or far infrared) and/or at least partof the UV range. The second interference filter or filters may inparticular carry out filtering in the infrared range, the firstinterference filter or filters being able to be dedicated to the visibleand/or UV range.

The Fabry-Perot cavities may be arranged on a transparent substrate, forexample comprising glass. Such a transparent substrate may allowintegration of the optical filtering device on a sensor, for example bytransfer onto a silicon substrate.

The optical filtering device may comprise only one structured layercommon to the first Fabry-Perot cavities in all the interference filtersof the device and arranged inside the first Fabry-Perot cavities. Such aconfiguration is advantageous since the use of only one structured layerper Fabry-Perot cavity simplifies and reduces the cost of the productioncompared with Fabry-Perot cavities comprising a plurality ofsuperimposed structured layers, and also avoids any degradation of thebottom structured layer when the top structured layer is produced. Inaddition, the presence of the spacer in the second interference filteror filters enables the optical filtering device to cover a spectralrange at least as extensive as the one covered by a filtering device notcomprising a spacer but using one or more superimposed structuredlayers.

The device according to the invention judiciously combines a pluralityof structured layers with one or more spacers within the sameinterference filter.

When the device is produced according to the first configuration, thatis to say when two distinct structured layers are used within the sameFabry-Perot cavity, this facilitates production of the device, inparticular the lithography steps that have to be used for producing thestructured layers, compared with a device that would compriseinterference filters carrying out similar filtering but which wouldcomprise only one structured layer. This is because, by distributing thestructurings in two superimposed structured layers, the constraints inproducing these structurings are lower than when the structurings haveto be produced in a single structured layer, for a given range ofspectra. In addition, this superimposition of structured layers makes itpossible to produce, for a given occupation surface, a larger number ofinterference filters, and therefore makes it possible to obtain a largernumber of transmission peaks in the filtering spectrum of the device.

When the device is produced according to the second configuration, thatis to say when two superimposed Fabry-Perot cavities are used to form aninterference filter, the levels of the transmission peaks of thefiltering spectrum of the device are more homogeneous with respect toone another, compared with a filtering device using only one Fabry-Perotcavity per interference filter. This second configuration does notconstitute a simple superimposition of a plurality of Fabry-Perotcavities. This is because, in this second configuration, the secondspacer present in the second cavity of the second filter is judiciouslyarranged under the third structured layer and is arranged so that itforms a planar surface with the elements around it so that the thirdstructured layer can be produced on this planar surface. The planenessof the third structured layer is also obtained, for the semi-reflectivelayer common to the two superimposed cavities and therefore makes itpossible then to produce the first cavity above the second cavity, therealso starting from a planar surface for producing the first structuredlayer.

The spacer may be arranged on or under the structured layer.

The first structured layer is advantageously continuous from oneFabry-Perot cavity to another. In other words, the main faces of theparts of the structured layer arranged in the various cavities arearranged in two planes only.

The spectral responses of the interference filters may cover a largesubstantially continuous spectral band (each interference filter beingable to form a band-pass filter passing a range of wavelengths adjacentto a range of wavelengths that another adjacent filter passes) or aplurality of distinct and not necessarily adjacent spectral bands.

The two materials with different refractive indices may be dielectricand/or semiconductor materials. This makes it possible to obtain betterspectral selectivity compared with the filtering devices using plasmonsvia the use of metal layers. Likewise, the first spacer may comprise atleast one dielectric or semiconductor material.

The first structured layer may comprise periodic patterns formed byportions of a second of the two materials with different refractiveindices arranged in a first of the two materials with differentrefractive indices.

The first structured layer and/or the second structured layer and/or thethird structured layer may comprise periodic patterns formedrespectively by the first and/or second and/or third portions of asecond of the two materials with different refractive indices arrangedin a layer of a first of the two materials with different refractiveindices formed respectively from the first and/or second and/or thirdportions of the first of the two materials with different refractiveindices.

In each of the first and/or second Fabry-Perot cavities, values ofdimensions (dimensions in a main plane of the structured layer) and of aperiod of the periodic patterns may be less than a value of a centrewavelength of the spectral response respectively of said first and/orsecond first-order 1 Fabry-Perot cavity.

The periodic patterns may form, in a main plane of the first structuredlayer, bidimensional structures, for example pads with a rectangular orsquare shape. The periodic patterns are in this case well suited tocarrying out filtering of a non-polarised light or of a light comprisingtwo polarisations.

In a variant, the periodic patterns may form, in the main plane of thefirst structured layer, unidimensional structures, for example slotsextending in only one direction. The periodic patterns are in this casewell suited to carrying out filtering of a light comprising a singlepolarisation.

Advantageously, the first and/or second and/or third portions of thesecond of the two materials with different refractive indices may beformed throughout the thickness respectively of the first structuredlayer and/or of the second structured layer and/or of the thirdstructured layer.

It is however possible for the portions of the second of the twomaterials with different refractive indices to be formed in only part ofthe thickness of the first structured layer. The same applies to thesecond and third structured layers. In this case, the presence of thespacer in the second interference filter or filters makes it possible tocover a large spectral band without necessarily having to use twostructured layers superimposed in Fabry-Perot cavities.

The optical filtering device may further comprise at least one firstetching stop layer arranged at least between one of the first and secondsemi-reflective layers and the first structured layer of the firstFabry-Perot cavity of the first interference filter. This etching stoplayer may in particular protect the structured layer when the spacer isproduced, which may involve a step of etching material present on thefirst interference filter or filters. This etching stop layer may havehigh etching selectivity compared with that of the material forming thespacer. A single etching stop layer may be sufficient to guarantee theintegrity of the structured layer common to all the filters. It ispossible for this etching stop layer to be present also at the secondinterference filter or filters, which simplifies the production of thisetching stop layer without causing disturbance in the filterings carriedout.

The first interference filter and/or the second interference filter maycomprise at least one second Fabry-Perot cavity superimposed on thefirst Fabry-Perot cavity. Compared with an interference filtercomprising only one Fabry-Perot cavity, the superimposition of twoFabry-Perot cavities, advantageously identical with respect to eachother, makes it possible to obtain better rejection of the filter and aspectral response where the flanks have a greater slope, and therefore amore precise range of wavelengths transmitted. Furthermore, thisconfiguration makes it possible to have greater uniformity, in terms ofmaximum transmission, of the spectral responses of the interferencefilters over the entire range of wavelengths sought.

In this configuration, one of the two semi-reflective layers of thefirst Fabry-Perot cavity may also form one of the two semi-reflectivelayers of the second Fabry-Perot cavity.

When the second interference filter comprises a second Fabry-Perotcavity, said second Fabry-Perot cavity may comprise at least one secondspacer arranged between a third semi-reflective layer and a secondstructured layer of said second Fabry-Perot cavity.

The second spacer may comprise at least one dielectric or semiconductormaterial.

The first spacer and the first etching stop layer may be arrangedbetween the first structured layer and the second semi-reflective layer,and:

-   -   according to the first configuration, a second etching stop        layer may be arranged between the first and second structured        layers;    -   according to the second configuration, a third etching stop        layer may be arranged between the third structured layer and the        third semi-reflective layer.

The optical filtering device may further comprise at least one portionof material absorbent vis-à-vis wavelengths with values lower than thatof a centre wavelength of a spectral response of the first Fabry-Perotcavity of the second interference filter, for example amorphous orpolycrystalline silicon, arranged on or in the first Fabry-Perot cavityof the second interference filter. This portion of material in this casemakes it possible to absorb some wavelengths transmitted at order 2 (orat the orders above 2) of the Fabry-Perot cavity of the secondinterference filter.

The first spacer and/or the second spacer may comprise amorphous orpolycrystalline silicon, which enables them also to fulfil the role ofabsorbent material as described above.

The optical filtering device may comprise a plurality of firstinterference filters arranged alongside one another and in whichproportions by volume of the two materials with different refractiveindices with respect to each other in the first structured layer and/orthe second structured layer and/or the third structured layer may bedifferent from one first interference filter to the other, and/or maycomprise a plurality of second interference filters arranged alongsideone another and in which proportions by volume of the two materials withdifferent refractive indices with respect to each other in the firststructured layer and/or the second structured layer and/or the thirdstructured layer may be different from one second interference filter tothe other.

The proportions by volume of the two materials with refractive indicesdifferent with respect to each other in the second interference filteror filters may be different from the proportions by volume of the twomaterials with refractive indices different with respect to each otherin the first interference filter or filters.

It is for example possible to have a plurality of first interferencefilters suitable for carrying out filtering in the visible range, andone or more second interference filters suitable for carrying outfiltering in the infrared range. It is possible to produce at least onefirst interference filter suitable for carrying out filtering in thevisible range and at least one second interference filter suitable forcarrying out filtering in the infrared range, in which the proportionsby volume of the two materials with different refractive indices withrespect to each other are similar in the first and second interferencefilters, the spacer or spacers solely present in the second filtermaking it possible to centre the second filter on a wavelength differentfrom that on which the first filter is centred.

The optical filtering device may comprise a total number of interferencefilters of between 5 and 15, or between 5 and 10. Such an opticalfiltering device may be integrated in a hyperspectral camera.

The interference filters of the optical filtering device may form amatrix of interference filters.

Each semi-reflective layer may comprise at least one metal material.

The invention also relates to an image sensor comprising at least oneoptical filtering device as defined previously, in which each of thefirst and second interference filters of the optical filtering device isarranged at one or more adjacent pixels of the image sensor.

The invention also relates to a method for producing an opticalfiltering device comprising at least first and second interferencefilters each comprising at least one first Fabry-Perot cavity,comprising at least the following steps:

-   -   producing a first semi-reflective layer of the first Fabry-Perot        cavities;    -   producing, on the first semi-reflective layer, a first        structured layer belonging conjointly to the first and second        interference filters, having a substantially constant thickness,        being substantially planar and comprising first portions of at        least two dielectric or semi-conductive materials, with        different refractive indices, intended to be arranged, in each        of the first Fabry-Perot cavities and in a plane parallel to the        first semi-reflective layer, alongside one another in        alternation;    -   producing at least one first spacer at a region of the first        structured layer intended to form part of the first Fabry-Perot        cavity of the second interference filter;    -   producing a second semi-reflective layer of the first        Fabry-Perot cavities;

a distance between the first and second semi-reflective layers of thefirst Fabry-Perot cavity of the second interference filter being greaterthan a distance between the first and second semi-reflective layers ofthe first Fabry-Perot cavity of the first interference filter,

and in which the first and second interference filters are producedaccording to a first configuration and/or a second configuration suchthat:

-   -   according to the first configuration, the method further        comprises, between the production of the first structured layer        and the production of the first spacer, the production of a        second structured layer intended to be arranged between the        first and second semi-reflective layers, belonging conjointly to        the first and second interference filters, having a        substantially constant thickness, being substantially planar and        comprising second portions of the two materials with different        refractive indices arranged, in each of the first Fabry-Perot        cavities and in the plane parallel to the first semi-reflective        layer, alongside one another in alternation;    -   according to the second configuration, the method further        comprises, before the first semi-reflective layer is produced,        the production of at least one second Fabry-Perot cavity of each        of the first and second interference filters, superimposed on        the first Fabry-Perot cavity and formed by the first and third        semi-reflective layers between which at least one third        structured layer is produced, the third structured layer        belonging conjointly to the first and second inference filters,        having a substantially constant thickness, being substantially        planar and comprising third portions of the two materials with        different refractive indices arranged, in each of the second        Fabry-Perot cavities and in the plane parallel to the first        semi-reflective layer, alongside one another in alternation, the        second Fabry-Perot cavity of the second interference filter        further comprising at least one second spacer arranged between        the third semi-reflective layer and the third structured layer        so that a distance between the first and third semi-reflective        layers of the second Fabry-Perot cavity of the second        interference filter is greater than a distance between the first        and third semi-reflective layers of the second Fabry-Perot        cavity of the first interference filter.

A method for producing an optical filtering device is also described,comprising at least first and second interference filters eachcomprising at least one first Fabry-Perot cavity, comprising at leastthe following steps:

-   -   producing a first semi-reflective layer of the first Fabry-Perot        cavities;    -   producing, on the first semi-reflective layer, a first        structured layer comprising at least two materials with        different refractive indices intended to be included in each of        the first Fabry-Perot cavities, the first structured layer being        common to the first and second interference filters and having a        substantially constant thickness;    -   producing at least a first spacer at a region of the first        structured layer intended to form part of the first Fabry-Perot        cavity of the second interference filter;    -   producing a second semi-reflective layer of the first        Fabry-Perot cavities;

a distance between the first and second semi-reflective layers of thefirst Fabry-Perot cavity of the second interference filter being greaterthan a distance between the first and second semi-reflective layers ofthe first Fabry-Perot cavity of the first interference filter.

This method makes it possible to simultaneously produce a plurality ofmultilayer interference filters with Fabry-Perot cavities, for examplearranged in a matrix, the resonance wavelengths of which cover aspectral range not limited by the indices of the materials used. Thismethod has the advantage of comprising few tricky steps, and the totalnumber of steps used remains low. This method may comprise only twolithography steps, including one at high definition (for producing thestructured layer), for producing at least two filters the spectralresponses of which may be distributed in the complete visible and nearinfrared range. Simply by modifying the mask used for the lithography ofthe layer to be structured, this method makes it possible to producemore filters positioned at intermediate wavelengths, for example morethan 11 interference filters.

This method may comprise steps implemented in thin-layer technology.

The production of the first structured layer may comprise theimplementation of the following steps:

-   -   deposition, on the first semi-reflective layer, of a layer of a        first of the two materials with different refractive indices;    -   lithography and etching of hollows in the layer of the first of        the two materials with different refractive indices, forming the        first portions of the first of the two materials with different        refractive indices;    -   deposition of a layer of a second of the two materials with        different refractive indices in the hollows and on the layer of        the first of the two materials with different refractive        indices;    -   planarisation of the layer of the second of the two materials        with different refractive indices with stoppage on the layer of        the first of the two materials with different refractive        indices, forming the first portions of the second of the two        materials with different refractive indices.

The production of the first spacer may comprise the implementation ofthe following steps:

-   -   deposition of a first etching stop layer on the first structured        layer or, when the first and second interference filters are        produced according to the first configuration, on the second        structured layer;    -   deposition, on the first etching stop layer, of a layer of        material intended to form the first spacer;    -   lithography and etching of the layer of material intended to        form the first spacer so that a remaining portion of said layer        of material forms the first spacer.

The method may further comprise, prior to the production of the firstsemi-reflective layer, the production of second Fabry-Perot cavitiessuperimposed on the first Fabry-Perot cavities.

In this case, when the first and second interference filters areproduced according to the second configuration, the production of thesecond Fabry-Perot cavities may comprise the implementation of thefollowing steps:

-   -   producing, at a first region of a substrate on which the first        interference filter is intended to be produced, a relief, the        thickness of which is substantially equal to that of the second        spacer intended to be produced;    -   depositing the third semi-reflective layer on the relief and on        a second region of the substrate on which the second        interference filter is intended to be produced;    -   producing the second spacer on a part of the third        semi-reflective layer intended to form part of the second        Fabry-Perot cavity of the second interference filter, the second        spacer and a part of the third semi-reflective layer intended to        form part of the second Fabry-Perot cavity of the first        interference filter forming a planar top surface;    -   producing, on said planar top surface, the third structured        layer;

and in which the first semi-reflective layer may next be produced on thethird structured layer.

When the first and second interference filters are produced according tothe first configuration, the production of the second structured layermay comprise the implementation of the following steps:

-   -   deposition of a second etching stop layer on the first        structured layer;    -   deposition, on the second etching stop layer, of a layer of a        first of the two materials with different refractive indices;    -   lithography and etching of hollows in the layer of the first of        the two materials with different refractive indices, forming the        second portions of the first of the two materials with different        refractive indices;    -   deposition of a layer of the second of the two materials with        different refractive indices in the hollows and on the layer of        the first of the two materials with different refractive        indices;    -   planarisation of the layer of the second of the two materials        with different refractive indices with a stoppage on the layer        of the first of the two materials with different refractive        indices, forming the second portions of the second of the two        materials with different refractive indices.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood better from a reading of thedescription of example embodiments given purely by way of indication andin no way limitatively with reference to the accompanying drawings, inwhich:

FIGS. 1 and 2 show schematically an optical filtering device accordingto first and second example embodiments;

FIG. 3 shows schematically an optical filtering device that is thesubject matter of the present invention, according to a firstembodiment;

FIG. 4 shows the spectral responses of an optical filtering deviceaccording to the first example embodiment;

FIGS. 5 to 10 show schematically steps of a method for producing anoptical filtering device according to the first example embodiment;

FIG. 11 shows schematically an optical filtering device that is thesubject matter of the present invention, according to a secondembodiment;

FIG. 12 shows the spectral responses of an optical filtering device thatis the subject matter of the present invention, according to the secondembodiment;

FIGS. 13 to 17 show schematically steps of a method for producing anoptical filtering device that is the subject matter of the presentinvention, according to the second embodiment;

FIG. 18 shows schematically an image sensor, also the subject matter ofthe present invention, according to a particular embodiment.

Identical, similar or equivalent parts of the various figures describedbelow bear the same numerical references so as to facilitate passagefrom one figure to another.

Various parts shown in the figures are not necessarily shown to auniform scale, in order to make the figures more legible.

The various possibilities (variants and embodiments) must be understoodas not being exclusive of one another and may be combined with oneanother.

DETAILED DISCLOSURE OF PARTICULAR EMBODIMENTS

Reference is made first of all to FIG. 1, which shows an opticalfiltering device 100 according to a first example embodiment.

The device 100 comprises a transparent substrate 102, comprising forexample glass. A first anti-reflective layer 104 is arranged on thesubstrate 102. The first anti-reflective layer 104 comprises for examplea dielectric material such as SiN. Its thickness is for exampleapproximately 50 nm, or more generally between approximately 10 nm and70 nm. The thickness of the layer 104 depends on the spectral rangesought by the device 100 and the refractive index of the layer 104.Different values, and in particular higher ones than those indicatedabove, may be envisaged since the anti-reflective effect is periodicwith the thickness of the layer 104. The layer 104 may be produced inthe form of a thin layer. A plurality of layers transparent to therequired wavelengths of the filters of the device 100, and withdifferent refractive indices, may be arranged on the substrate 102,stacked one on top of the other, such a multilayer being able to form ananti-reflective element more effective than a single anti-reflectivelayer. In a variant, the device 100 may not comprise any anti-reflectivelayer.

Interference filters 106 with Fabry-Perot cavities (six filtersreferenced 106.1-106.6 in FIG. 1) are arranged on the firstanti-reflective layer 104. The six filters 106.1-106.6 are such that thecentre wavelengths of the spectral responses of these filters aredifferent with respect to one another and are called respectivelyλ_(106.1)-λ_(106.6). In general terms, the device 100 may comprise atleast two interference filters 106, and advantageously between 5 and 15filters 106, or between 5 and 10 filters 106, or even an even largernumber of filters 106. The number of filters 106 that the device 100 hasdepends on the number of distinct spectral responses required in thespectral range that is to be processed by the device 100. Although inFIG. 1 the filters 106 are arranged alongside one another along the axisy, the filters 106 of the device 100 are generally arranged in the formof a matrix of filters.

These filters 106 comprise a first semi-reflective layer 108, orsemi-reflective mirror, which is here common to all the filters 106. Thefirst semi-reflective layer 108 is a metal layer, comprising for examplesilver, and the thickness of which is for example approximately 44 nm ormore generally between approximately 30 nm and 60 nm. The firstanti-reflective layer 104 arranged between the substrate 102 and thefirst semi-reflective layer 108 prevents or limits light reflections onthe first semi-reflective layer 108.

The filters 106 also comprise a structured layer 110 forming part of theenvironment of the filters 106 located between the semi-reflectivelayers of the filters 106. This structured layer 110 is common to allthe filters 106 and has a thickness e_(N) that is substantially constantfor all the filters 106. The structured layer 110 comprises at least twomaterials with different refractive indices n_(B) and n_(H), heredielectric materials corresponding to SiN (index n_(H)) and SiO₂ (indexn_(B)), these two materials being structured so that the various regionsof the structured layer 110 present in the various filters 106 comprisevarious proportions by volume of these two materials so that the actualrefractive index of the structured layer 110 varies from one filter toanother. The material with the larger refractive index n_(H) is referredto as the first material, and is here SiN, and the one with the smallerrefractive index n_(B) is referred to as the second material, and ishere SiO₂. The materials of the structured layer 110 are transparent atleast vis-à-vis wavelengths intended to be transmitted by the filters106. In a variant, at least one of the first and second materials may bea semiconductor material.

In the example in FIG. 1, a region 112.1 of the structured layer 110forming part of the filter 106.1 comprises only the second material. Aregion 112.2 of the structured layer 110 forming part of the filter106.2 comprises the first material, in which the hollows 114 areproduced throughout the thickness of the structured layer 110 and arefilled by portions of the second material, thus forming structurings ofthe structured layer 110. In the example described here, these hollows114, and therefore the portions of the second material, each have across section, in a main plane of the structured layer 110, that is tosay a plane parallel to the face of the structured layer 110 locatedagainst the first semi-reflective layer 108 (parallel to the plane (X,Y)in FIG. 1), with a rectangular or square shape. The period of thestructuring is less than the value of the centre wavelength of thespectral response of the filter 106.2. The structurings of thestructured layer 110 may have forms other than rectangular or square,for example groove or trench shapes produced over the entire length orthe entire width of the filter. A region 112.3 of the structured layer110 forming part of the filter 106.3 comprises only the first material.The filters 106.4, 106.5 and 106.6 each comprise a region 112.4, 112.5and 112.6 of the structured layer 110 that are here similar to theregions 112.1, 112.2 and 112.3 respectively.

An etching stop layer 116 is arranged on the structured layer 110. Thisetching stop layer 116 comprises a material which can be etched muchmore slowly than the materials of the spacer 120 described below, forexample AlN or TiO₂, and which is transparent vis-à-vis the wavelengthsintended to be transmitted by the filters 106. The thickness of theetching stop layer 116 is for example between approximately 5 nm and 10nm. This etching stop layer 116 is present in the filtering device 100because of the production method used and which is described below inrelation to FIGS. 5 to 10.

For the filters 106.1 to 106.3, the transparent materials locatedbetween the semi-reflective layers of these filters correspond to thematerials of the regions 112.1 to 112.3 of the structured layer 110 andto the material of the etching stop layer 116. Thus, at these filters106.1 to 106.3, a second semi-reflective layer 118 is arranged directlyon the etching stop layer 116. Thus the incident light at the filters106.1 to 106.3 is reflected between the semi-reflective layers 108 and118 in the structured layer 110 and the etching stop layer 116. Theheight, or thickness, of the Fabry-Perot cavities of the filters 106.1to 106.3 formed between the two semi-reflective layers 108 and 118 isequal to the sum of the thickness e_(N) of the structured layer 110 andthe thickness of the etching stop layer 116.

For the filters 106.4 to 106.6, the second semi-reflective layer 118 isarranged not directly on the etching stop layer 116 but on a spacer 120corresponding here to a portion of dielectric material with refractiveindex n_(S) and thickness e_(S), arranged between the etching stop layer116 and the second semi-reflective layer 118. The height, or thickness,of the Fabry-Perot cavities of the filters 106.4 to 106.6 formed betweenthe two semi-reflective layers 108 and 118 is therefore different fromthe height of the filters 106.1 to 106.3 because of the presence of thespacer 120, thus modifying the values of the centre wavelengthsλ_(106.4) to λ_(106.6) of the spectral responses of these filterscompared with those of the centre wavelengths λ_(106.1) to λ_(106.3) ofthe spectral responses of the filters 106.1 to 106.3. This thickness isequal to the sum of the thickness e_(N) of the structured layer 110, thethickness of the etching stop layer 116 and the thickness e_(S) of thespacer 120. The material of the spacer 120 corresponds for example toone of the materials of the structured layer, advantageously the onewith the lowest index n_(B), or to any other dielectric or semiconductormaterial. The spacer 120 comprises a material that is transparentvis-à-vis the wavelengths intended to be transmitted by the filters106.4 to 106.6. In the example in FIG. 1, the spacer 120 comprises SiO₂.The incident light to the filters 106.4 to 106.6 is reflected betweenthe semi-reflective layers 108 and 118 in the structured layer 110, theetching stop layer 116 and the spacer 120.

In general terms, the device 100 comprises at least one filter, theheight, or thickness, of which is different from one or more otherfilters of the device 100 because of the presence of a spacer 120 inthis filter.

A second anti-reflective layer 122 is arranged on the secondsemi-reflective layer 118 in all the filters 106, and prevents or limitslight reflections on the second semi-reflective layer 118. This secondanti-reflective layer 122 has for example a thickness similar to that ofthe layer 104. A plurality of second anti-reflective layers 122 may bearranged on the second semi-reflective layer 118.

Thus the centre wavelengths of the spectral responses of the filters 106of the device 100 are defined both by the thickness of the Fabry-Perotcavities of the filters 106 that differ in the device 100, and by theeffective index of the medium between the semi-reflective layers, whichchanges from one filter to another within the device 100 by virtue ofthe structured layer 110.

The thickness e_(N) of the structured layer 110 is defined according tothe previously described equation (1) (the stop layer 116 has littleinfluence on the filtering which is carried out and, for thecalculations carried out using equation (1), it may be considered, as afirst approximation, to be absent from the filters 106). Thus thisthickness e_(N) may be chosen by considering the characteristics of thefirst filter 106.1 the wavelength λ_(106.1) of which has the smallestvalue among those of the centre wavelengths of the spectral responses ofthe filters 106 of the filtering device 100, that is to say according tothe values of λ_(106.1) and of the index n_(B) of the second material,which is the only one present in the region 112.1 of the structuredlayer 110 of the first filter 106.1. The wavelength λ_(106.3), that isto say the longest centre wavelength of the spectral responses among thefilters that do not comprise the spacer 120, depends on the thicknesse_(N) and the index n_(H) of the first material, which is the only onepresent in the region 112.3 of the structured layer 110 of the filter106.3. For the filters that do not comprise the spacer 120 and whichcomprise regions of the structured layer 100 comprising the structuringformed by the first and second materials (the filter 106.2 in theexample in FIG. 1), the dimensions of the structurings, corresponding tothe dimensions of the hollows 114, can be calculated as described in thedocument US 2011/0290982 A1.

The value of the thickness e_(S) of the space 120 is chosen byconsidering the characteristics of the filter 106.4 the wavelengthλ_(106.4) of which has the smallest value among those of the centrewavelengths of the spectral responses of the filters that comprise thespacer 120, that is to say according to values of λ_(106.4) and of theindex n_(B) of the second material, which is the only one present in theregion 112.4 of the structured layer 110 of the filter 106.4, and alsoaccording to the refractive index n_(S) of the material of the spacer120. The previously described equation (1) can be used for calculatingthis thickness e_(S), the numerator of this equation corresponding tothe sum of the optical paths in each of the layers 110 and 120, that isto say 2n_(S)e_(S)+2n_(B)e_(N) (as before, for reasons of simplificationof the calculations carried using equation (1), the etching stop layer116 is considered, as a first approximation, to be absent from thefilters 106 because of the small impact of this layer on the filteringscarried out). The value of the wavelength λ_(106.6), that is to say thelongest centre wavelength of the spectral responses among the filtersthat comprise the spacer 120, depends on the thicknesses e_(N) and e_(S)and of the index n_(H) of the first material, which is the only onepresent in the region 112.6 of the structured layer 110 of the filter106.6 (the optical paths in question are optical paths in each of thelayers 110 and 120, that is to say 2n_(S)e_(S)+2n_(H)e_(N)). For thefilters that comprise the spacer 120 and which comprise regions of thestructured layer 100 comprising structurings (the filter 106.5 in theexample in FIG. 1), the dimensions of the structurings can be calculatedas described in the document US 2011/0290982 A1. These dimensions aregenerally greater than the centre wavelength of the spectral response ofthe filter comprising the structurings.

The design of the device 100 may be such that λ_(106.3)=λ_(106.4) sothat the spectral ranges covered by the two groups of filters (firstgroup of filters 106.1-106.3 that do not comprise the spacer 120 and thesecond group of filters 106.4-106.6 that comprise the spacer 120) arecontiguous and result in a single spectral range covering a widespectrum. It is however possible that this is not the case.

The changes to the spectral responses caused by the Fresnel reflectionat the interface between the structured layer 110 and the spacer 120,and by the etching stop layer 116 in the cavities of the filters 106,are in general not significant and can be minimised or optimisedjudiciously by conventional methods of simulating multilayer stackshaving recourse to software using multilayer optimisation algorithmsbased on the Abeles formalism such as the needles method as describedfor example in the document “Application of the needle optimizationtechnique to the design of optical coatings” by A. V. Tikhonravov et al,Applied Optics, vol. 35, n° 28, pages 5493-5508, 1 Oct. 1996.

In the first example embodiment described above, the spacer 120 isarranged between the second semi-reflective layer 118 and the structuredlayer 110. In a variant, the spacer 120 may be arranged between thefirst semi-reflective layer 108 and the structured layer 110, with inthis case a relief previously formed on the substrate 102, as describedbelow in relation to FIGS. 11 to 18.

The etching stop layer 116 is present at least in the filters 106 thatdo not comprise the spacer 120. For reasons of simplification of design,the etching stop layer may be present in all the filters 106, as is thecase with the example in FIG. 1. The etching stop layer 116 is arrangedon or under the structured layer 110 depending on whether the spacer 120is arranged on or under the structured layer 110.

The semi-reflective layers 108 and 118 preferably comprise at least onemetal. The refractive index of a metal is complex and can be denotedn+ik. The metal forming the semi-reflective layers 108 and 118 ispreferably chosen so that the ratio k/n is as high as possible, forexample at least equal to approximately 10, throughout the spectralrange covered by the interference filters 106, in order to obtain goodtransmission of order 1 wavelengths and good rejection of thewavelengths at higher orders, which is the case with silver.

In a variant, the filters 106.4 to 106.6 may comprise a plurality ofspacers 120 formed by one or more materials transparent to thewavelengths intended to be transmitted by the filters 106.4-106.6.

In a variant of the first example embodiment, the device 100 maycomprise, in addition to the filters 106.1 to 106.6, other filtersformed from semi-reflective layers 108 and 118, of the structured layer110, but which comprise one or more spacers such that the height, orthickness, of the Fabry-Perot cavities of these filters is differentfrom those of the filters 106.1 to 106.6. The device 100 may comprise aplurality of groups of filters 106 each including one (or more) spacerwith a different thickness and/or different material or materials. It isin particular possible for all the groups of filters (each group offilters corresponding to the filters having the same thickness) tocomprise a spacer. The presence of a spacer in all the filters 106 canimprove adhesion when the second semi-reflective layer 118 is depositedon these spacers, compared with a deposition of the secondsemi-reflective layer 118 directly on the etching stop layer 116.

In the first example embodiment described above, the hollows 114 areproduced throughout the thickness of the structured layer 110. In asecond example embodiment shown in FIG. 2, the hollows 114 are producedthrough a part of the thickness of the structured layer 110. Thus thefirst material with refractive index n_(H) is also present under theportions of the second material with refractive index n_(B) filling thehollows 114. The fact that the hollows 114 pass through only part of thethickness of the structured layer 110 means that, in order to obtain agiven effective refractive index at a region of the structured layer 110that comprises these hollows (corresponding to the regions 112.2 and112.5 in FIG. 2), the lateral dimensions of the hollows 114, that is tosay the dimensions in the plane (X,Y), are greater than those of hollowsmaking it possible to obtain this same effective refractive index butwhich would be produced throughout the thickness of the structured layer110.

In the two example embodiments described above, in each group of filters106, only one filter (the filter 106.2 for the group of filters notcomprising the spacer 120, and the filter 106.5 for the group of filterscomprising the spacer 120) comprises structurings. It is howeverpossible for, in each group of filters, a plurality of filters, or evenall the filters, to comprise structurings of different dimensions inorder to obtain different spectral responses.

In the two example embodiments described above, the device 100 comprisesonly one structured layer 100 common to all the filters 106 and arrangedbetween the semi-reflective layers 108 and 118 of these filters 106.FIG. 3 shows the device 100 according to a first embodiment comprisingten filters 106.1 to 106.10, each of these filters comprising, betweenthe two semi-reflective layers 108 and 118, two parts of two structuredlayers 110.1 and 110.2 arranged one on the other. The filters 106.1 to106.5 form the first group of the filters not comprising the spacer 120,and the filters 106.6 to 106.10 form the second group of filterscomprising the spacer 120.

The filter 106.1 comprises regions 112.11 and 112.21 of the structuredlayers 110.1 and 110.2 comprising only the second material. The filter106.2 comprises a region 112.12 of the first structured region 110.1comprising only the second material, and a region 112.22 of the secondstructured layer 110.2 comprising the first material in which hollows114.2 are produced throughout the thickness of the second structuredlayer 110.2 and are filled by the second material. The filter 106.3comprises regions 112.13 and 112.23 of the structured layers 110.1 and110.2 comprising the first material in which hollows 114.1 and 114.2 areproduced and filled by the second material. The filter 106.4 comprises aregion 112.14 of the first structured layer 110.1 comprising only thefirst material, and a region 112.24 of the second structured layer 110.2comprising the first material in which hollows 114.2 are producedthroughout the thickness of the second structured layer 110.2 and arefilled by the second material. Finally, the filter 106.5 comprisesregions 112.15 and 112.25 of the structured layers 110.1 and 110.2comprising only the first material. The regions 112.16 to 112.20 and112.26 to 112.30 of the structured layers 110.1 and 110.2 in the filters106.6 to 106.10 are similar to those of the filters 106.1 to 106.5.

A first etching stop layer 116.1 is arranged on the second structuredlayer 110.2. The function of this first etching stop layer 116.1 issimilar to that previously described for the etching stop layer 116.

A second etching stop layer 116.2 comprising a material which can beetched much more slowly than the first material of the second structuredlayer 110.2 is interposed between the structured layers 110.1 and 110.2.This second etching stop layer 116.2 makes it possible not to damage thefirst structured layer 110.1 when the second structured layer 110.2 isproduced, particularly during the etching of the first material of thesecond structured layer 110.2. This second etching stop layer 116.2comprises for example a material with a similar nature to that of thefirst etching stop layer 116.1, such as AlN or TiO₂, and which istransparent vis-à-vis the wavelengths intended to be transmitted by thefilters 106. The thickness of the second etching stop layer 116.2 is forexample between approximately 2 nm and 10 nm.

Thus the effective index in the regions of the structured layers 110.1and 110.2 differ, in each group of filters, from one filter to another,which makes it possible to produce filters having different spectralresponses. The combination of the regions of a plurality of superimposedstructured layers therefore makes it possible to produce a large numberof regions with different effective refractive indices, since theaccessible lateral dimensions of the structures are limited by thetechnological possibilities. On the other hand, the method for producingthe filtering device according to the first embodiment is more complexthan for the production of a filtering device comprising only onestructured layer as in the first and second example embodiments.However, this configuration of the filtering device with twosuperimposed structured layers facilitates the lithography carried outcompared with that used during the production of a filtering devicecomprising the same number of filters, with similar spectral filteringranges, but formed in a single structured layer.

As in the second example embodiment, the hollows produced in thestructured layers 110.1 and 110.2 may be produced through only part ofthe thickness of these layers.

FIG. 4 shows spectral responses (that is to say the value of thecoefficient of transmission T as a function of the wavelength, innanometres in FIG. 4) obtained for a filtering device 100 comprisingonly one structured layer 110 in which the structurings are producedthroughout the thickness of the structured layer 110. This filteringdevice comprises six filters 106 not comprising the spacer 120 and thespectral responses of which correspond to the curves referenced 10, 12,14, 16, 18 and 20, and five filters comprising the spacer 120 and thespectral responses of which correspond to the curves referenced 22, 24,26, 28 and 30.

The filtering device 100 for obtaining the spectral responses shown inFIG. 4 comprises the following elements:

-   -   glass substrate 102;    -   first anti-reflective layer 104 comprising SiN and with a        thickness of 50 nm;    -   first semi-reflective layer 108 comprising Ag and with a        thickness of 44 nm;    -   structured layer 110 with a thickness e_(N) of 105 nm, the        second material of which is SiO₂ and the first material of which        is SiN, and comprising structurings, and therefore hollows 114,        with a rectangular shape;    -   spacer 120 comprising SiO₂ and with a thickness e_(S) of 80 nm;    -   second semi-reflective layer 118 comprising Ag and with a        thickness of 50 nm;    -   second anti-reflective layer 122 comprising SiN and with a        thickness of 42 nm.

The SiN used in this filtering device is enriched with silicon, whichconfers a relatively high refractive index on it, close to that of TiO₂,with on the other hand a certain absorption of the short wavelengths(which has no impact in the present case since the filters with aspectral response located in the blue range comprise little or no SiN).

With regard to the filters comprising the spacer 120, the secondanti-reflective layer 122 is covered with a portion of amorphous siliconwith a thickness for example equal to 15 nm. This portion of amorphoussilicon makes it possible, with regard to the short wavelengthscorresponding approximately to those of the blue colour, to attenuate“bounces” or secondary peaks of the spectral responses of the filterscomprising the spacer 120 and the centre wavelengths of which arelonger. These bounces are caused by the orders higher than the order 1of the Fabry-Perot cavities of these filters. The portion of amorphoussilicon is transparent in the remainder of the spectral range. Thisportion of amorphous silicon also makes it possible, by constructiveinterferences, to increase the transmission of the filters in which theportion of amorphous silicon is located.

The eleven filters of this filtering device cover a spectral band ofbetween approximately 450 nm and 900 nm. The centre wavelengths of thespectral responses (order 1) are uniformly distributed in this spectralband, which covers the major part of the visible and near infraredspectrum. The sixth interference filters not comprising the spacer 120and the spectral responses of which correspond to the curves referenced10, 12, 14, 16, 18 and 20 cover a first part of this spectral bandranging from approximately 450 nm to approximately 680 nm. The fiveinterference filters comprising the spacer 120 and the spectralresponses of which correspond to the curves referenced 22, 24, 26, 28and 30 cover a second part of this spectral band ranging fromapproximately 720 nm to approximately 900 nm.

The relative proportions by volume of SiO₂ and SiN in the regions of thestructured layer 110 in these various filters are indicated in thefollowing table. In this table, the filters are identified by the centrewavelength of their spectral response. The widths of the SiN pads in theregions of the structured layer 110, the widths of spaces between thesepads and the period of these pads are also indicated in this table.

Centre Relative Spacer Width of Width of wavelength proportion thicknessSiN pad spaces between Period (nm) SiO₂/SiN (%) (nm) (nm) pads (nm) (nm)450 100/0   0 0 0 0 500 85/15 0 135 215 350 540 70/30 0 195 155 350 58050/50 0 250 100 350 630 30/70 0 290 60 350 680  0/100 0 0 0 0 720 85/1580 135 215 350 750 70/30 80 195 155 350 800 50/50 80 250 100 350 84030/70 80 290 60 350 900  0/100 80 0 0 0

FIGS. 5 to 10 show steps of a method for producing the device 100previously described in relation to FIG. 1.

The first anti-reflective layer 104 is first of all deposited on thesubstrate 102, and then the first semi-reflective layer 108 is depositedon the first anti-reflective layer 104. A layer 124 comprising the firstmaterial and with a thickness equal to the thickness e_(N) of thestructured layer 110 intended to be produced is next deposited on thefirst semi-reflective layer 108 (FIG. 5). Advantageously, prior to thedeposition of the layer 124, the first semi-reflective layer 108 can beencapsulated by a fine protective layer (not shown in the figures) inorder to prevent degradation of the metal of the first semi-reflectivelayer 108 by air or by the etching of the layer 124 carried outsubsequently.

Steps of lithography and etching of the layer 124 are next implementedin order to form the hollows 114 at the regions of the structured layerintended to comprise structurings, and thus eliminate the parts of thelayer 124 located at the regions of the structured layer 110 intendednot to comprise the first material (FIG. 6). When these hollows 114 passthrough the entire thickness of the layer 124, as is the case in theexample described here, the first semi-reflective layer 108 serves as anetching stop layer when the hollows 114 are etched.

As shown in FIG. 7, a layer 126 comprising the second material and witha thickness at least equal to the thickness e_(N) of the structuredlayer 110, and generally equal to two or three times the thickness e_(N)in order to facilitate the implementation of the following polishing, isnext deposited on the previously produced structure, in the etched partsof the layer 124 (that is to say in the hollows 114 and at the regionsof the structured layer 110 intended to comprise only the secondmaterial). Parts of the layer 126 are also deposited on the remainingparts of the layer 124.

Chemical mechanical polishing (CMP) is next carried out with stoppage onthe remaining portions of the layer 124, thus eliminating the parts ofthe layer 126 deposited on the remaining parts of the layer 124 (FIG.8), and forming the structured layer 110.

As shown in FIG. 9, the etching stop layer 116 is next deposited on thestructured layer 110, and then a layer 127 comprising the material ofthe spacer 120 and with a thickness e_(S) is deposited on the etchingstop layer 116.

Steps of lithography and etching of the layer 127 are next implementedso that a remaining portion of the layer 127 forms the space 120 (FIG.10). The presence of the etching stop layer 116 at the etched part orparts of the layer 127 prevents over-etching in the structured layer 110during the etching of the layer 127.

The second semi-reflective layer 118 is next deposited on the whole ofthe structure, that is to say on the spacer 120 and on the part or partsof the etching stop layer 116 not covered by the spacer 120. A fineadhesion layer (not shown) may be deposited on the whole of thestructure, prior to the deposition of the second semi-reflective layer118. The second anti-reflective layer 122 is next deposited on thesecond semi-reflective layer 118. The device obtained corresponds to thedevice 100 shown in FIG. 1.

FIG. 11 shows the filtering device 100 according to a second embodimentin which each interference filter 106.1-106.6 comprises two Fabry-Perotcavities placed one above the other.

The device 100 comprises the first anti-reflective layer 104 arranged onthe substrate 102. The thickness of the part of the firstanti-reflective layer 104 formed in the first group of filters106.1-106.3 not comprising a spacer is greater than that of the part ofthe first anti-reflective layer 104 formed at the second group offilters 106.4-106.6. A third semi-reflective layer 128 is arranged onthe first anti-reflective layer 104. At the second group of filters106.4-106.6, a second spacer 120.2 is produced on the thirdsemi-reflective layer 128. The first anti-reflective layer 104 thusforms, at a first region of the substrate 102 on which the first groupof filters 106.1-106.3 is intended to be produced, a relief, thethickness of which is substantially equal to that of the second spacer120.2. The sum of the thicknesses of the third semi-reflective layer 128and of the part of the first anti-reflective layer 104 in the firstgroup of filters 106.1-106.3 is therefore substantially equal to that ofthe thicknesses of the second spacer 120.2, of the third semi-reflectivelayer 128 and of the part of the first anti-reflective layer 104 in thesecond group of filters 106.4-106.6. In a variant, this relief could beproduced by hollowing out the substrate 102, the first anti-reflectivelayer 104 being in this case able to have a constant thickness.

A top face of the third semi-reflective layer 128 in the first group offilters 106.1-106.3 and a top face of the second spacer 120.2 form aplanar surface on which another etching stop layer 116.3 is arranged,referred to as the third etching stop layer in order to distinguish itfrom the second etching stop layer 116.2 previously described inrelation to FIG. 3, the role of which is to protect the second spacer120.2.

The device 100 comprises another structured layer 110.3, referred to asthe third structured layer in order to distinguish it from the secondstructured layer 110.2 previously described in relation to FIG. 3,common to all the filters 106.1-106.6. The third structured layer 110.3comprises the two materials with different refractive indices and, insome regions, structurings similar to those previously described inrelation to FIG. 1. The third structured layer 110.3 is here similar tothe structured layer 110 previously described in relation to FIG. 1.

The first semi-reflective layer 108, comprising for example a materialsimilar to that of the third semi-reflective layer 128, is arranged onthe third structured layer 110.3. Thus, for each of the filters106.1-106.6, a second Fabry-Perot cavity is formed between the twosemi-reflective layers 108 and 128. First Fabry-Perot cavities, similarto those of the device 100 previously described in relation to FIG. 1,are next produced on the second Fabry-Perot cavities.

The first structured layer 110.1 common to all the filters 106.1-106.6is arranged on the first semi-reflective layer 108. The first structuredlayer 110.1 is similar to the third structure layer 110.3.

The first etching stop layer 116.1 is arranged on the first structuredlayer 110.1.

The first spacer 120.1, for example similar to the second spacer 120.2,is arranged on the first etching stop layer 116.1 at the second group offilters 106.4-106.6. The second semi-reflective layer 118 is arranged onthe first spacer 120.1 and, in the first group of filters 106.1-106.3,on the first etching stop layer 116.1.

The second anti-reflective layer 122, for example similar to the onepreviously described in relation to FIG. 1, is arranged on the secondsemi-reflective layer 118. In each of the filters 106.1-106.6, the twoFabry-Perot cavities formed may be similar with respect to one another.

Compared with the interference filters of the device 100 according tothe first example embodiment, those of the device 100 according to thesecond embodiment have better rejection and better selectivity.

Because the production of each of the structured layers 110.1 and 110.3requires the implementation of a planarisation step, for example by CMP,each structured layer 110.1 and 110.3 is produced on a planar face (thetop face of the third etching stop layer 116.3 for the third structuredlayer 110.3 and the top face of the first semi-reflective layer 108 forthe first structured layer 110.1). For this purpose, the second spacer120.2 is arranged under the third structured layer 110.3. Thus, afterthe production of the second Fabry-Perot cavities of the filters106.1-106.6, the surface on which the first Fabry-Perot cavities of thefilters 106.0-106.6 are produced is planar.

FIG. 12 shows the spectral responses obtained for a filtering devicesimilar to the one described in relation to FIG. 11, that is to saycomprising two structured layers 110.1 and 110.3 in which thestructurings are produced throughout the thickness and forming, for eachfilter, two Fabry-Perot cavities placed one above the other. Thisfiltering device comprises six interference filters not comprising thespacers 120.1 and 120.2 and the spectral responses of which correspondto the curves referenced 40, 42, 44, 46, 48 and 50, and fiveinterference filters comprising the spacers 120.1 and 120.2 and thespectral responses of which correspond to the curves referenced 52, 54,56, 58 and 60.

The filtering device 100 making it possible to obtain the spectralresponses shown in FIG. 12 comprises the following elements:

-   -   glass substrate 102;    -   first anti-reflective layer 104 comprising SiN and with a        thickness of 20 nm in the five filters comprising the spacers,        and a thickness of 100 nm in the six filters not comprising the        spacers;    -   third semi-reflective layer 129 comprising Ag and with a        thickness of 27 nm;    -   second spacer 120.2 comprising SiO₂ and with a thickness of 80        nm;    -   third structured layer 110.3 with a thickness e_(N) of 105 nm,        the second material of which is SiO₂ and the first material of        which is SiN, and comprising rectangular-shaped hollows 114;    -   first semi-reflective layer 108 comprising Ag and with a        thickness of 61 nm;    -   first structured layer 110.1 with a thickness e_(N) of 105 nm,        the second material of which is SiO₂ and the first material of        which is SiN, and comprising rectangular-shaped hollows 114;    -   first spacer 120.1 comprising SiO₂ and with a thickness e_(S) of        80 nm;    -   second semi-reflective layer 118 comprising Ag and with a        thickness of 27 nm;    -   second anti-reflective layer 122 comprising SiN and with a        thickness of 67 nm.

As in the example embodiment previously described, the SiN used in thisfiltering device is enriched with silicon and, in the filters comprisingthe spacers 120.1 and 120.2, the second anti-reflective layer 122 iscovered with a portion of amorphous silicon with a thickness of 120 nm.Furthermore, the first anti-reflective layer 104 is suited to theoptical impedance of all the filters 106.1-106.6 at the wavelength ofinterest of each of these filters.

The eleven filters of this filtering device cover a spectral bandbetween approximately 450 nm and 900 nm. The centre wavelengths of thespectral responses (first order) are uniformly distributed in thisspectral band, which covers the major part of the visible spectrum andnear infrared. The six interference filters not comprising the spacers120.1 and 120.2 and the spectral responses of which correspond to thecurves referenced 40, 42, 44, 46, 48 and 50 cover a first part of thisspectral band ranging from approximately 450 nm to approximately 680 nm.The five interference filters comprising the spacers 120.1 and 120.2 andthe spectral responses of which correspond to the curves referenced 52,54, 56, 58 and 60 cover a second part of this spectrum band ranging fromapproximately 720 nm to approximately 900 nm. Compared with the spectralresponses shown in FIG. 4, those shown in FIG. 12 have maximumamplitudes that are more homogeneous with respect to one another, byvirtue of the superimposition of the two Fabry-Perot cavities in each ofthe filters 106.

FIGS. 13 to 17 show steps of a method for producing the filtering device100 previously described in relation to FIG. 11.

A first layer 130, from which the first anti-reflective layer 104 isintended to be produced, is deposited on the substrate 102 (FIG. 13).The thickness of this first layer 130 is equal to the thickness of thesecond spacer 120.2 intended to be produced.

As shown in FIG. 14, lithography and etching of this first layer 130 arenext implemented in the first layer 130 so that a remaining portion 132of the first layer 130 is intended to form a part of the firstanti-reflective layer 104 located in the filters 106.1-106.3 notcomprising spacers.

The first anti-reflective layer 104 is next completed by depositing amaterial similar to that of the first layer 130 both on the remainingportion 132 of the first layer 130 and on the part of the substrate 102not covered by the remaining portion 132, with a thickness equal to thatof the part of the first anti-reflective layer 104 intended to belocated at the filters intended to comprise the spacers 120.1 and 120.2.

The third semi-reflective layer 128 is next deposited on the firstanti-reflective layer 104. A layer intended to form the second spacer120.2, that is to say comprising the material of this second spacer120.2 and the thickness of which is at least equal to that of the secondspacer 120.2, is next deposited on the third semi-reflective layer 128.A planarisation of the CMP type is next implemented with stoppage on thepart of the third semi-reflective layer 128 (or of a fine protectivelayer, not shown) located on the part of the first anti-reflective layer104 with the greatest thickness. The remaining portion of this layerforms the second spacer 120.2 (FIG. 15). The second spacer 120.2 and thepart of the third semi-reflective layer 128 intended to form part of thesecond Fabry-Perot cavity of the first interference filters 106.1-106.3form a planar top surface.

As shown in FIG. 16, the third etching stop layer 116.3 is nextdeposited on the structure previously produced, that is to say on theplanar surface formed by the second spacer 120.2 and the part of thethird semi-reflective layer 128 located on the part of the firstanti-reflective layer 104 with the greatest thickness.

A layer 134 comprising the first material and with a thickness equal tothe thickness e_(N) of the third structured layer 110.3 intended to beproduced is next deposited on the third etching stop layer 116.3.

Steps of lithography and etching of the layer 134 are next implementedin order to form the hollows 114, that is to say to eliminate the partsof the layer 134 located at the regions of the third structured layer110.3 intended not to comprise the first material. A layer comprisingthe second material and with a thickness at least equal to the thicknesse_(N) of the third structured layer 110.3 is next deposited on thepreviously produced structure, in the etched parts of the layer 134(that is to say in the hollows 114 and at the regions of the thirdstructured layer 110.3 intended to comprise only the second material).Parts of this layer are also deposited on the remaining parts of thelayer 134.

A chemical mechanical polishing (CMP) is next carried out with stoppageon the remaining portions of the layer 134, thus eliminating the partsof the layer deposited on the remaining parts of the layer 134 andforming the third structured layer 110.3 (FIG. 17).

The first semi-reflective layer 108 is next deposited on the thirdstructured layer 110.3, and then the first structured layer 110.1 isnext produced by implementing steps similar to those producing the thirdstructured layer 110.3. The device 100 is next completed by theimplementation of steps similar to those previously described forproducing the device 100 according to the first example embodiment.

In the implementations and example embodiments described above, thematerials of the structured layers 110, 110.1 and 110.2 and of thespacers 120, 120.1 and 120.2 are dielectric materials. In a variant, oneor more of these materials may be semiconductor materials, for exampleamorphous or polycrystalline silicon, ZnO, ZnS, ZnSe or ZnTe.

FIG. 18 shows schematically an image sensor 1000 according to aparticular embodiment.

The image sensor 1000 comprises an electronic part 1002, formed forexample by detection elements of the CMOS type forming pixels 1004. Thefiltering device 100 is integrated on the front face of this electronicpart 1002, so that the filters 106 are arranged opposite the pixels1004. It is possible for each filter 106 to be arranged opposite a pixel1004, or opposite a plurality of adjacent pixels. The image sensor 1000may be a hyperspectral camera, and may comprise other elements, forexample optical and electronic, such as electrical interconnections andmicrolenses, not shown in FIG. 18.

The invention claimed is:
 1. An optical filtering device, comprising: at least first and second interference filters each including at least one first Fabry-Perot cavity formed by first and second semi-reflective layers between which at least one first structured layer is arranged, wherein the first structured layer belongs conjointly to the first and second interference filters, has a substantially constant thickness, is substantially planar and comprises first portions of at least two dielectric or semiconductor materials, with different refractive indices, arranged, in at least one of the first Fabry-Perot cavities and in a plane parallel to the first semi-reflective layer, alongside one another in alternation, the first Fabry-Perot cavity of the second interference filter comprises at least one first spacer arranged between one of the first and second semi-reflective layers and the first structured layer in such a way that a distance between the first and second semi-reflective layers of the first Fabry-Perot cavity of the second interference filter is greater than a distance between the first and second semi-reflective layers of the first Fabry-Perot cavity of the first interference filter, the first and second interference filters are produced such that: the first and second interference filters each comprise at least one second Fabry-Perot cavity placed on top of the first Fabry-Perot cavity and formed by the first and a third semi-reflective layer between which at least one second structured layer is arranged, the second structured layer belonging conjointly to the first and second interference filters, having a substantially constant thickness, being substantially planar and comprising second portions of the two materials with different refractive indices arranged, in at least one of the second Fabry-Perot cavities and in the plane parallel to the first semi-reflective layer, alongside one another in alternation, the second Fabry-Perot cavity of the second interference filter further comprising at least one second spacer arranged between the third semi-reflective layer and the second structured layer so that a distance between the first and third semi-reflective layers of the second Fabry-Perot cavity of the second interference filter is greater than a distance between the first and third semi-reflective layers of the second Fabry-Perot cavity of the first interference filter, and the optical filtering device comprises a plurality of said first interference filters arranged alongside one another and wherein proportions by volume of the two materials with different refractive indices with respect to each other, in at least one of the first structured layer and the second structured layer, are different from one first interference filter to the other, or comprises a plurality of said second interference filters arranged alongside one another and wherein proportions by volume of the two materials with different refractive indices with respect to each other in at least one of the first structured layer and the second structured layer, are different from one second interference filter to the other.
 2. The optical filtering device according to claim 1, wherein at least one of the first structured layer and the second structured layer comprises periodic patterns formed respectively by at least one of the first and second portions of a second of the two materials with different refractive indices arranged in a layer of a first of the two materials with different refractive indices formed respectively from at least one of the first and second portions of the first of the two materials with different refractive indices.
 3. The optical filtering device according to claim 2, wherein, in at least one of the first and second Fabry-Perot cavities, values of dimensions and of a period of the periodic patterns are less than a value of a center wavelength of a spectral response respectively of said at least one of the first and second Fabry-Perot cavities at first order.
 4. The optical filtering device according to claim 2, wherein said at least one of the first and second portions of the second of the two materials with different refractive indices are formed throughout the thickness respectively of said at least one of the first structured layer and the second structured layer.
 5. The optical filtering device according to claim 1, further comprising: at least one first etching stop layer arranged at least between one of the first and second semi-reflective layers and the first structured layer in the first Fabry-Perot cavity of the first interference filter.
 6. The optical filtering device according to claim 1, wherein the first spacer and the first etching stop layer are arranged between the first structured layer and the second semi-reflective layer, a second etching stop layer is arranged between the second structured layer and the third semi-reflective layer.
 7. The optical filtering device according to claim 1, further comprising: at least one portion of material absorbent vis-à-vis wavelengths with values less than that of a center wavelength of a spectral response of the first Fabry-Perot cavity of the second interference filter, arranged on or in the first Fabry-Perot cavity of the second interference filter.
 8. An image sensor comprising at least one optical filtering device according to claim 1, wherein each of the first and second interference filters of the optical filtering device is arranged at one or more adjacent pixels of the image sensor.
 9. An optical filtering device, comprising: at least first and second interference filters each comprising at least one first Fabry-Perot cavity formed by first and second semi-reflective layers between which at least one first structured layer is arranged, wherein the first structured layer belongs conjointly to the first and second interference filters, has a substantially constant thickness, is substantially planar and comprises first portions of at least two dielectric or semiconductor materials, with different refractive indices, arranged, in each of the first Fabry-Perot cavities and in a plane parallel to the first semi-reflective layer, alongside one another in alternation, the first Fabry-Perot cavity of the second interference filter comprises at least one first spacer arranged between one of the first and second semi-reflective layers and the first structured layer in such a way that a distance between the first and second semi-reflective layers of the first Fabry-Perot cavity of the second interference filter is greater than a distance between the first and second semi-reflective layers of the first Fabry-Perot cavity of the first interference filter, and the first and second interference filters are produced such that: the first and second interference filters each comprise at least one second Fabry-Perot cavity placed on top of the first Fabry-Perot cavity and formed by the first and a third semi-reflective layer between which at least one second structured layer is arranged, the second structured layer belonging conjointly to the first and second interference filters, having a substantially constant thickness, being substantially planar and comprising second portions of the two materials with different refractive indices arranged, in each of the second Fabry-Perot cavities and in the plane parallel to the first semi-reflective layer, alongside one another in alternation, the second Fabry-Perot cavity of the second interference filter further comprising at least one second spacer arranged between the third semi-reflective layer and the second structured layer so that a distance between the first and third semi-reflective layers of the second Fabry-Perot cavity of the second interference filter is greater than a distance between the first and third semi-reflective layers of the second Fabry-Perot cavity of the first interference filter, and the second structured layer is arranged above the first structured layer.
 10. A method for producing an optical filtering device comprising at least first and second interference filters each comprising at least one first Fabry-Perot cavity, the method comprising: producing a first semi-reflective layer of the first Fabry-Perot cavities; producing, on the first semi-reflective layer, a first structured layer belonging conjointly to the first and second interference filters, having a substantially constant thickness, being substantially planar and comprising first portions of at least two dielectric or semiconductor materials, with different refractive indices, intended to be arranged, in at least one of the first Fabry-Perot cavities and in a plane parallel to the first semi-reflective layer, alongside one another in alternation; producing at least one first spacer at a region of the first structured layer intended to form part of the first Fabry-Perot cavity of the second interference filter; and producing a second semi-reflective layer of the first Fabry-Perot cavities, wherein a distance between the first and second semi-reflective layers of the first Fabry-Perot cavity of the second interference filter being greater than a distance between the first and second semi-reflective layers of the first Fabry-Perot cavity of the first interference filter, the first and second interference filters are produced so that: the method further comprises, before the first semi-reflective layer is produced, the production of at least one second Fabry-Perot cavity of each of the first and second interference filters, superimposed on the first Fabry-Perot cavity and formed by the first and a third semi-reflective layers between at least one second structured layer is produced, the second structured layer belonging conjointly to the first and second inference filters, having a substantially constant thickness, being substantially planar and comprising second portions of the two materials with different refractive indices arranged, in at least one of the second Fabry-Perot cavities and in the plane parallel to the first semi-reflective layer, alongside one another in alternation, the second Fabry-Perot cavity of the second interference filter further comprising at least one second spacer arranged between the third semi-reflective layer and the second structured layer so that a distance between the first and third semi-reflective layers of the second Fabry-Perot cavity of the second interference filter is greater than a distance between the first and third semi-reflective layers of the second Fabry-Perot cavity of the first interference filter, and a plurality of said first interference filters are made alongside one another and wherein proportions by volume of the two materials with different refractive indices with respect to each other, in at least one of the first structured layer and the second structured layer, are different from one first interference filter to the other, or wherein a plurality of said second interference filters are made alongside one another and wherein proportions by volume of the two materials with different refractive indices with respect to each other in at least one of the first structured layer and the second structured layer, are different from one second interference filter to the other.
 11. The method according to claim 10, wherein the production of the first structured layer comprises: depositing, on the first semi-reflective layer, of a layer of a first of the two materials with different refractive indices; lithography and etching of hollows in the layer of the first of the two materials with different refractive indices, forming the first portions of the first of the two materials with different refractive indices; depositing a layer of a second of the two materials with different refractive indices in the hollows and on the layer of the first of the two materials with different refractive indices; and planarization of the layer of the second of the two materials with different refractive indices with stoppage on the layer of the first of the two materials with different refractive indices, forming the first portions of the second of the two materials with different refractive indices.
 12. The method according to claim 10, wherein the production of the first spacer comprises: depositing a first etching stop layer on the first structured layer depositing, on the first etching stop layer, a layer of material intended to form the first spacer; and lithography and etching of the layer of material intended to form the first spacer so that a remaining portion of said layer of material forms the first spacer.
 13. The method according to claim 10, wherein the production of the second Fabry-Perot cavities comprises: producing, at a first region of a substrate on which the first interference filter is intended to be produced, a relief, the thickness of which is substantially equal to that of the second spacer intended to be produced; depositing the third semi-reflective layer on the relief and on a second region of the substrate on which the second interference filter is intended to be produced; producing the second spacer on a part of the third semi-reflective layer intended to form part of the second Fabry-Perot cavity of the second interference filter, the second spacer and a part of the third semi-reflective layer intended to form part of the second Fabry-Perot cavity of the first interference filter forming a planar top surface; and producing, on said planar top surface, the second structured layer, wherein the first semi-reflective layer may next be produced on the second structured layer. 